It is well known that certain ore bodies containing significant amounts of certain sulfides and pyrites, such as iron pyrite, may be reactive with ammonium nitrate or other nitrate salts. In some instances the heat produced in a borehole from the reaction between these ores and explosives containing nitrate salts has caused premature detonations. Frequently, these reactive ores are associated with geothermal regions that can produce high temperatures in boreholes. In addition, boreholes drilled into the ores can become hot due to the reaction of newly exposed ore in the boreholes with air (oxygen). The resulting high temperatures further enhance the reactivity of the ore with nitrate-based explosives.
U.S. Bureau of Mines Reports of Investigation Nos. 7187 and 8373 detail studies in which urea was added to AN or ANFO to repress the reaction with certain sulfide and pyrite containing ores or with ores containing weathering products of these constituents such as ferrous sulfate. Up to 5% urea was reportedly needed to prevent any reaction of the AN with reactive ores. U.S. Pat. No. 3,447,982 discloses the addition of up to 1% powdered urea with a Stengel type AN (crystallized and flaked) to suppress the reaction of such AN with reactive ores. In some cases, the urea was included in the presolidified AN melt. Greater than 1% powdered urea was to be avoided because of increased shock sensitivity. U.S. Pat. No. 3,708,356 extended this approach to porous AN or ANFO wherein up to 1% powdered urea was added to suppress reaction with reactive ores.
It has recently been observed that emulsion explosives are also reactive with some ores, this despite the fact that the oil continuous phase of a stable emulsion serves as a barrier to help reduce direct contact of the internal phase nitrate salts and the ore. It has been found in the present invention that the inclusion of a substantial amount of urea in solution with the nitrate salts of the internal phase of the emulsion greatly reduces or even eliminates reaction of the explosive with the ore.
Water-in-oil emulsion explosives are well-known in the art. They are fluid when formed (and can be designed to remain fluid at temperatures of use) and are used in both packaged and bulk forms. They commonly are mixed with ammonium nitrate prills and or ANFO to form a "heavy ANFO" product, having higher energy and, depending on the ratios of components, better water resistance than ANFO. Such emulsions normally are reduced in density by the addition of air voids in the form of hollow microspheres, other solid air entraining agents or gas bubbles, which materially sensitize the emulsion to detonation. A uniform, stable dispersion of the air entraining agent or gas bubbles is important to the detonation properties of the emulsion. Gas bubbles, if present, normally are produced by the reaction of chemical gassing agents.